In cellular and land-mobile wireless systems, multi-element antenna receive diversity is commonly used to improve uplink performance. Improvements in downlink performance are also desirable, especially for data traffic, since load is expected to be greater for data in the downlink, than in the uplink (e.g., web-browsing scenario). One solution, receive diversity at the mobile handset, is not generally feasible due to limitations of space and cost.
Multi-element antenna transmit diversity implemented at the base station is, however, desirable in this case. Delay transmit diversity is one option, but its performance is significantly sub-optimal, and it may actually degrade performance in time-dispersive propagation media. An alternative space-time transmit diversity technique, developed by Siavash Alamouti in “A simple transmit diversity technique for wireless communications,” IEEE Journal on Selected Areas in Communications, October 1998, offers full two-branch diversity gain in slowly-varying frequency-flat fading channels, and achieves the Shannon capacity of a multi-element antenna channel that has a two-element transmitter and a single-element receiver, as shown by Joseph Thomas in “On the capacity achieved by space-time block codes,” Research Report, Institute for Systems Research, MD, May 1999. Alamouti's proposal consists of encoding the transmitted sequence in doublets such that the transmissions from the two antennas are mutually orthogonal and a linear demodulation scheme where the decision rule for maximum likelihood detection reduces to a comparison of distance metrics.
The scheme is described in FIG. 1. The elements of the transmitting antenna are indexed ‘0’ and ‘1’. If s0 and s0 are transmitted in succession from antenna ‘0’ at signaling intervals n and n+1 respectively then −s1* and s0* are transmitted during these signaling intervals from antenna ‘1’. This is a slight modification (made for convenience) of, though essentially equivalent to, the originally proposed version. If h(0) and h(1) are, respectively, the flat fading channel responses of the path from antennas ‘0’ and ‘1’ to the receiver (assumed to consist of a single-element antenna) then received mixtures at time n and n+1 arex0=h(0)s0−h(1)s1*+v0x1=h(0)s1+h(1)s0*+v1where v0 and v1 are noise samples, and * denotes complex conjugate. The total transmitted power is fixed at the same level as for the case of a transmitter with a single-element antenna, i.e., each of the two transmit antenna elements is allocated half the total transmitter power and the corresponding amplitude factors, are absorbed into the channel responses h(0) and h(1) above. The mixtures x0 and x1 are readily decoupled as below:r0=h(0)*x0+h(1)x1*=(|h(0)|2+|h(1)|2)s0+w0r1=h(0)*x1−h(1)x0*=(|h(0)|2+|h(1)|2) s1+w1where w0=h(0)*v0+h(1)v1* and w1=h(0)*v1−h(1)v0*. The maximum likelihood detector reduces to choosing si if(|h(0)|2+|h(1)|2−1)|si|2+∥rj, si∥2≦(|h(0)|2+|h(1)|2−1)|sk|2+∥rj, sk∥2, ∀(i≠k)The fact that transmissions on the two channels are mutually orthogonal is central to permitting a linear decoupling scheme at the demodulator. However, this does not work for practical (i.e., time-dispersive) channels since the orthogonality imposed by the Alamouti modulation technique is destroyed by such dispersivity.
Other known prior art includes patent application Ser. No. 09/624,453, entitled “Method and Device for Exploiting Transmit Diversity in Time Varying Wireless Communication Systems.” This prior art extends Alamouti's method to work in time-dispersive channels, and contains some aspects in common with the present invention (chiefly, the use of time-reversed, conjugated symbol blocks on the second antenna) but is restrictive, on two counts. First, it requires data sub-blocks to be separated with either cyclic or zero prefixes, following the context of orthogonal frequency division multiplexing (OFDM) systems. Therefore it is not readily applicable to systems employing training-based channel estimation, such as the GSM cellular system. Second, it stipulates that equalization be accomplished in the frequency domain via linear processing. It is desirable to allow more robust time-domain equalization methods such as decision feedback equalization (DFE) and maximum likelihood sequence estimation (MLSE) to be used in addition to linear equalization. In sum therefore, the Alamouti scheme does not work for wideband systems where the propagation media are time-dispersive.